A spate of earthquakes across the middle of the U.S. is almost certainly" man-made, and may be caused by wastewater from oil or gas drilling injected into the ground, U.S. government scientists said in a study. Researchers from the U.S. Geological Survey said that for the three decades until 2000, seismic events in the nation's midsection averaged 21 a year. They jumped to 50 in 2009, 87 in 2010 and 134 in 2011. Those statistics, included in the abstract of a research paper to be discussed at the Seismological Society of America conference next week in San Diego, will add pressure on an energy industry already confronting more regulation of the process of hydraulic fracturing. An abstract of the federal study, which was led by William Ellsworth, Earthquake Science Center staff director for the U.S. Geological Survey in Menlo Park, California, was published online earlier this month. "A naturally-occurring rate change of this magnitude is unprecedented outside of volcanic settings or in the absence of a main shock, of which there were neither in this region," Ellsworth and his colleagues wrote.